Dark matter is one of the biggest problems in contemporary physics, astronomy, and cosmology. Basically it is matter that neither emits nor absorbs light, but it appears to make up the majority of the matter in the universe. This has been a growing problem in astronomy since the middle of the last century. The trouble is that there appears to be a great deal of mass in the universe as well as the stars and galaxies that can be seen through telescopes, but it is not clear exactly what this non-luminous material could be.

Evidence for Dark Matter

There is evidence for dark matter on all scales of the universe:

In the Galaxy

Just as 19th Century mathematicians predicted the existence of the planet Neptune by observing the pull of its gravitation field on the orbits of other planets, modern scientists have deduced the existence of dark matter by looking at the motion of stars in our galaxy. The velocity with which stars orbit the centre of the galaxy can be determined by measuring their 'red shift'. Visible stars are concentrated in the centre of the galaxy. Therefore if the mass of the galaxy was only made up of these stars, we would expect the velocity of stars to fall as we move out into the galactic disk. Further away from a central mass they should not need to move so fast to stay in orbit. But instead they travel at roughly the same speed as stars near the galactic centre. This suggests that there is a great deal of extra mass in the disk in addition to the stars there; the galaxy appears to be surrounded by a halo of dark matter. The velocity of stars on the fringe of the galaxy is so high that without the gravitational pull of the extra matter, they would fly off into outer space.

In Clusters of Galaxies

A galaxy cluster is a group of galaxies held together by their own gravity. However, when we measure the speed with which each galaxy moves, it appears a lot more gravity is required to hold the cluster together than can be explained by the stars we can see. There must be a lot of dark matter there that we can't see.

In the universe

By measuring the rate at which our universe is expanding, it appears we live in a flat universe, where the rate at which it has been expanding since the Big Bang, is reduced by its own gravity. However if we measure the total amount of light in the universe, and multiply this by the mass-to-light ratio for a typical star, then the value we get isn’t nearly enough to hold the universe together. Incredibly, dark matter accounts for only 21% of the density of the universe. The remainder is made up by the equally mysterious dark energy1.

So dark matter is needed everywhere to hold the universe together, without it the spiral galaxies would unravel, clusters would just fly apart; in fact galaxies and planets would never have formed, the universe would just be an expanding cloud of hot gas.

So What Is It?

There are various theories about exactly what dark matter is:

MACHOs – Black Holes, Planets and Dead Stars

There are of course a great many known objects in the galaxy that do not emit light. Planets, black holes, and dead or very dim stars could all account for some dark matter. These are known to astronomers as MACHOs (Massive Astrophysical Compact Halo Objects). While these objects may not emit any light, there are various tricks that allow astronomers to detect them by measuring how they focus the light from non-dark matter behind them. For the last few decades groups at observatories around the world have searched the sky for these objects. But although many have been found, there do not seem to be nearly enough to explain all the missing matter. It appears that the majority of dark matter is made up of something much more exotic.

Non-baryonic Matter – Neutrinos and WIMPs

This fits other theories in cosmology. Cosmologists study the evolution of the universe since the Big Bang. How it expanded to its present size, and how the initial hot gas of elementary particles cooled to form atoms, molecules, and eventually stars and planets. Their work suggests that of all the matter formed, only a small fraction should consist of the atoms and molecules we are familiar with – so called baryonic matter. The remainder consists of strange non-baryonic particles.

Particle physicists already know of one breed of non-baryonic particle, the neutrino. This is a curious, lightweight (but not massless) particle that moves at very close to the speed of light and hardly interacts with ordinary matter at all. Billions pass straight through the Earth every day. While they could account for some dark matter in the universe, they are probably too light to explain it all, and as they move much too fast to orbit a galaxy. It seems that galactic dark matter is made of something else.

Fortunately physicists have predicted the existence of other types of non-baryonic particles, but most of these have not yet been directly detected. These include a class of particles of particular interest to dark matter scientists, called WIMPs (Weakly Interacting Massive Particles).

As WIMPs are weakly interacting, like neutrinos they only very rarely hit an ordinary atom; and as they are massive particles, possibly as heavy as a uranium atom they could account for the remaining dark matter in the galaxy. As other proposed explanations for missing matter have not been found, WIMPs are now the principle candidate for galactic dark matter. So far, nobody has ever detected one.

WIMP Hunting

So, it looks as if a large fraction of the matter in our universe is made up of WIMPs. But so far this is only a theory. This means that trying to directly detect a WIMP is a hot topic in contemporary physics, and the thought of a potential Nobel Prize, has helped many professors think up new experiments to achieve this.

WIMP hunting basically involves building a particle detector, and waiting for a WIMP to hit it. The interaction rate is believed to be very low, maybe once a year in a 1 tonne detector, and it is very difficult to distinguish between a WIMP and another particle such as a neutron. Therefore WIMP search experiments are located in deep underground laboratories where the background radiation is greatly reduced.

Theoretical physicists claim that the experimentalists will find WIMPs within the next few years. They have been saying this for many years.

1 Dark energy is an intrinsic density of vacuum space; it also contributes an intrinsic pressure which causes the expansion of the universe to accelerate. The first evidence for dark energy came from observations of this effect, which cannot be explained by dark matter alone.

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